The maize genome was successfully transformed with transgenes in the early 1990's. Over the last twenty years, numerous methodologies have been developed for transforming the maize genome, wherein a transgene is stably integrated into the maize genome. This evolution of maize transformation methodologies has resulted in the capability to successfully introduce a transgene comprising an agronomic trait within the maize genome. The introduction of insect resistance and herbicide tolerant traits within maize plants in the late—1990's provided producers with a new and convenient technological innovation for controlling insects and a wide spectrum of weeds, which was unparalleled in cultivation farming methods. Currently, transgenic maize is commercially available throughout the world, and new transgenic maize products such as Enlist™ Corn offer improved solutions for ever-increasing weed challenges. The utilization of transgenic maize in modern agronomic practices would not be possible, but for the development and improvement of maize transformation methodologies.
However, current maize transformation methodologies rely upon the random insertion of transgenes within the maize genome. Reliance on random insertion of genes into a genome has several disadvantages. The transgenic events may randomly integrate within gene transcriptional sequences, thereby interrupting the expression of endogenous traits and altering the growth and development of the maize plant. In addition, the transgenic events may indiscriminately integrate into locations of the corn genome that are susceptible to gene silencing, culminating in the reduced or complete inhibition of transgene expression either in the first or subsequent generations of transgenic maize plants. Finally, the random integration of transgenes within the maize genome requires considerable effort and cost in identifying the location of the transgenic event and selecting transgenic events that perform as designed without agronomic impact to the plant. Novel assays must be continually developed to determine the precise location of the integrated transgene for each transgenic event, such as a maize event. The random nature of maize transformation methodologies results in a “position-effect” of the integrated transgene, which hinders the effectiveness and efficiency of maize transformation methodologies.
Targeted genome modification of plants has been a long-standing and elusive goal of both applied and basic research. Targeting genes and gene stacks to specific locations in the Zea mays genome will improve the quality of transgenic events, reduce costs associated with production of transgenic events and provide new methods for making transgenic plant products such as sequential gene stacking. Overall, targeting trangenes to specific genomic sites is likely to be commercially beneficial. Significant advances have been made in the last few years towards development of methods and compositions to target and cleave genomic DNA by site specific nucleases (e.g., Zinc Finger Nucleases (ZFNs), Meganucleases, Transcription Activator-Like Effector Nucelases (TALENS) and Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated nuclease (CRISPR/Cas) with an engineered crRNA/tracr RNA), to induce targeted mutagenesis, induce targeted deletions of cellular DNA sequences, and facilitate targeted recombination of an exogenous donor DNA polynucleotide within a predetermined genomic locus. See, for example, U.S. Patent Publication No. 20030232410; 20050208489; 20050026157; 20050064474; and 20060188987, and International Patent Publication No. WO 2007/014275, the disclosures of which are incorporated by reference in their entireties for all purposes. U.S. Patent Publication No. 20080182332 describes use of non-canonical zinc finger nucleases (ZFNs) for targeted modification of plant genomes and U.S. Patent Publication No. 20090205083 describes ZFN-mediated targeted modification of a plant EPSPs genomic locus. Current methods for targeted insertion of exogenous DNA typically involve co-transformation of plant tissue with a donor DNA polynucleotide containing at least one transgene and a site specific nuclease (e.g., ZFN) which is designed to bind and cleave a specific genomic locus of an actively transcribed coding sequence. This causes the donor DNA polynucleotide to stably insert within the cleaved genomic locus resulting in targeted gene addition at a specified genomic locus comprising an actively transcribed coding sequence.
An alternative approach is to target the transgene to preselected target nongenic loci within the corn genome. In recent years, several technologies have been developed and applied to plant cells for the targeted delivery of a transgene within the corn genome. However, much less is known about the attributes of genomic sites that are suitable for targeting. Historically, non-essential genes and pathogen (viral) integration sites in genomes have been used as loci for targeting. The number of such sites in genomes is rather limiting and there is therefore a need for identification and characterization of targetable optimal genomic loci that can be used for targeting of donor polynucleotide sequences. In addition to being amenable to targeting, optimal genomic loci are expected to be neutral sites that can support transgene expression and breeding applications. A need exists for compositions and methods that define criteria to identify optimal nongenic loci within the corn genome for targeted transgene integration.